The present disclosure relates to a light guide for linearly irradiating light of a light source while guiding the light, an image reading apparatus for reading an image of a document to which light from a light guide is irradiated, and an image forming apparatus for forming an image on a sheet based on an image read by the image reading apparatus.
Conventionally, an illumination device for illuminating a document with light of an LED (Light Emitting Diode) using a rod-like light guide is known as an illumination device for a scanner device for reading an image of a document. This illumination device reflects light by a reflecting member formed on one side surface of the light guide. Further, the reflected light is taken out in a direction orthogonal to the light guide to linearly illuminate a document in a main scanning direction. In such an illumination device, light from the LED is emitted to the outside only by being reflected once near the LED. As a result, there is a problem that illuminance is high near the LED while being low at positions distant from the LED.
Thus, the LED is so arranged that the center position thereof is displaced from the center of the reflecting member on the side surface of the light guide. The illumination device is known which has improved illuminance unevenness in the main scanning direction by reducing the emission of the light from the LED to the outside by being reflected only once.
In the illumination device as described above, not only illuminance unevenness in the main scanning direction, but also alight distribution quality in a sub scanning direction differs due to a difference in the number of light reflections on the inner surface of the light guide. Specifically, near the LED where the number of light reflections on the inner surface of the light guide is small, light is irradiated in a range narrow in the sub scanning direction. On the other hand, at positions which are distant from the LED and where the number of light reflections on the inner surface of the light guide is large, light is irradiated while being diffused in a range wide in the sub scanning direction.
Light emitted from the light guide is irradiated to a document placed on a transparent document platen or the like. However, if the document is lifted from the document platen, a distance between the light guide and the document differs depending on the position in the main scanning direction. A degree of an illuminance change when the distance between the light guide and the document changes is affected by light distribution in the sub scanning direction by the light guide. Thus, if light distribution in the sub scanning direction differs depending on the position in the main scanning direction as described above, the document may be lifted. In this case, a change of the illuminance corresponding to the position in the main scanning direction increases, wherefore there has been an inconvenience that illuminance unevenness caused by the lift of a document increases.